In the exhaust system of an internal combustion engine of a motor vehicle, there is arranged a gas sensor for sensing the concentration of a specific component (e.g., the concentration of oxygen) in the exhaust gas which is the measurement gas.
Such a gas sensor has a gas sensor element built therein. The gas sensor element includes, for example, a solid electrolyte body having oxygen ion conductivity, a measurement gas-side electrode provided on one surface of the solid electrolyte body, a reference gas-side electrode provided on the other surface of the solid electrolyte body, and a porous diffusion-resistant layer which covers the measurement gas-side electrode and which allows the measurement gas to permeate therethrough. The gas sensor element is used in a state of having been heated to a high temperature (e.g., not lower than 500° C.) at which the solid electrolyte body can be activated. Therefore, when water condensate contained in the exhaust gas intrudes into the gas sensor element and adheres to the high-temperature solid electrolyte body, a large thermal shock may be applied to the solid electrolyte body, causing water-induced cracking to occur.
To solve the above problem, there is disclosed, for example in Patent Document 1, a gas sensor element which has a structure for improving the resistance to water damage. Specifically, the gas sensor element has its outer peripheral surface covered with a protective layer; the protective layer is formed of a porous material which is permeable to the exhaust gas and dense. By setting the surface roughness Ra of the protective layer to be lower than or equal to 3.0 μm, the protective layer is hydrophilic at room temperature and water-repellent at high temperature. Consequently, it is possible to prevent the adherence of water droplets to the gas sensor element at high temperature, thereby suppressing water-induced cracking from occurring.